1. Field of the Invention
The invention relates to a method for determining errors in manufacturing systems, especially errors of relative alignment between various elements in a manufacturing system. The invention further relates to a method for constructing an error detection system. The method of the present invention finds application in lithography systems where errors involving mask alignment, focus, magnification, etc. may be determined. Another aspect of the invention relates to a feedback control loop for sensing an error in a lithography system, determining the cause(s) of the error, and correcting the cause(s) of the error.
2. Description of the Related Art
a. SCALPEL lithography
FIG. 1 schematically depicts a SCALPEL (SCattering with Angular Limitation in Projection Electron Lithography) process. In general, the SCALPEL approach employs the principle of electron scattering to delineate circuit patterns on substrates. A mask 20 is used to shape an electron beam source. Mask 20 includes electron transmissive regions 22 and 24 having different electron scattering properties. Region 22 is a pattern of high electron scattering material examples of which include high atomic number metals such as gold and tungsten. Region 24 is a low electron scattering material, for example, a low atomic number element or compound such as silicon or silicon nitride formed into a membrane.
As electron beam 10 traverses the mask, electrons are scattered, the amount and angle of scattering being a function of the atomic number of the constituent atoms in the material. As a result of the differential scattering properties of the mask, an angular distribution of electrons is formed at the exit surface of the mask. Those electrons 12 having passed through patterned region 22 of high atomic weight material are generally scattered to a higher angle than those electrons 14 which passed only through the low atomic number membrane region 24.
The scattered electrons 12 and 14 pass into projection lens system 30 which demagnifies the image formed by the scattered electrons. In the back focal plane of the projecting lens system, the electrons are distributed by their angle of scatter. A filter 40 having an aperture 42 is placed in the back focal plane to angularly select electrons which will form the ultimate image. If aperture 42 is sufficiently small and on the focal axis, only those electrons scattered through small angles will contribute to the final image 52 on substrate 50. The electrons scattered through small angles 12, i.e., electrons which passed through the low atomic number membrane regions 24, are the electrons which interact with a substrate material, such as a resist, to create a latent image. For a given mask and optical system, the contrast in the image is determined by the size of the angularly limiting aperture.
SCALPEL lithography is further described in Berger et al., J. Vac. Sci. Technol., B9, November/December 1991, pp. 2996-2999 and U.S. Pat. Nos. 5,079,112 and 5,130,213, the disclosures of which are incorporated by reference herein.
b. System Errors
In SCALPEL lithography, as in numerous other manufacturing operations, the relationship of the various system components to one another is important to the success of the resultant process. During the printing process, the image of features on the mask must be precisely aligned with wafer features that are present from previous levels of processing. The allowable error for level-to-level overlay is by convention less than 1/3 of the minimum feature size and combines alignment error with errors from other machine and process effects. Among these other errors are optical errors, e.g., focus, magnification, astigmatism, that need to be monitored and minimized during wafer exposure. For submicron features, alignment of the mask image and substrate features must be extremely precise, in the range of nanometers.
One approach to the problem of system alignment for SCALPEL lithography involves scanning the demagnified image of a mask feature over an identically-shaped mark that is fabricated on a wafer and measuring the intensity of electrons which are backscattered by the substrate mark. The substrate marks are fabricated from a material having a higher backscatter coefficient than the surrounding substrate material. An example is to use mask features and wafer marks consisting of identical lines and spaces as described in Farrow et al., J. Vac. Sci. Technol. B, 10(6) November/December 1992, pp. 2780-2783, the disclosure of which is incorporated by reference herein. An extremum of backscattered electron intensity occurs at the point where a beam of electrons formed by the mask feature is exactly aligned with lines on the substrate. In this position, the mask image is correctly aligned with the substrate.
While the above-described method provides a useful alignment technique, during mark detection it is desirable to generate a signal that is particularly suited for the derivation of specific information whether it be alignment or other system conditions such as focus and magnification. In the above-described mark detection method the signal is a function of the overlap between the mask image and the wafer mark. Therefore, the mark pattern geometry determines the intensity profile of the signal and likewise the suitability for specific information requirements. Intensity profile refers to the measured signal intensity, e.g., backscattered electron or other signal, as a function of the adjustable parameter, e.g., relative alignment.
Thus, there is a need in the art for a precise error detection system to determine various sources of system error resulting, e.g., from misalignment or misadjustment of various manufacturing system components. There is a further need in the art for an error detection system which generates a signal explicitly constructed to yield specific information concerning various system parameters.